Automatic gain control circuit



70 If: JOVACZ June 8 1937. w. K

R OCH 2,083,026 AUTOMATIC GAIN CONTROL CIRCUIT Originai Filed July 6, 1934 2 Sheets-Sheet 1 imam/kc:

70 ,4. (ISM/RC5 70 Al/O/O NE INVENTOR.

INFIELD R. KOCH ATTORNEY.

June 8, 1937. w. R. KOCH 2,083,026

AUTOMATIC GAIN CONTROL CIRCUIT Original Filed July 6, 1954 2 Sheets-Sheet 2 NETWORK K GR/DS 70 A. l c 92 INVENTOR.

. WINFIELD R. KOCH ATTORNEY.

Patented June 8, 1937 UNITED STATES PATENT OFFICE AUTOMATIC GAIN CONTROL CIRCUIT Winfield R. Koch, Merchantville, N. J., assignor to Radio Corporation of America, a corporation of Delaware 3 Claims.

My present invention relates to gain control circuits, and more particularly to such circuits when utilized for automatic Volume control; the present application being a division of my application Serial No. 733,987, filed July 6, 1934.

One of the main objects of the present inven tion is to provide automatic gain control circuits for amplifiers, whether of high frequency or audible frequency, which circuits embody control tubes having output electrodes adapted to be energized by alternating current energy, the alternating current energy being of audible frequency.

Another important object of the invention is to provide automatic volume control circuits for radio receivers wherein a bridge network, energize-d by alternating current, has its balance upset when signals are impressed upon the receiving system, and the unbalance current is utilized to provide the AVG voltage of the receiver.

Still other objects of the invention are to improve generally the efficiency of automatic gain control circuits for radio receivers, and more especially to provide in radio receivers automatic volume control circuits, and in some instances interchannel noise suppressor circuits, which are not only reliable in operation, but readily assembled in a radio receiver.

The novel features which I believe to be characteristic of my invention are set forth in particularity in the appended claims. The invention itself, however, both as to its. organization and method of operation, will best be understood by reference to the following description taken in connection with the drawings, in which I have indicated diagrammatically several circuit organizations whereby my invention may be carried into effect. 1

In the drawings:

Fig. 1 shows a form of automatic volume control circuit utilizing a bridge control network,

Fig. 2 shows a modification of the arrangement in Fig. 1,

Fig. 3 shows still another modification of the arrangement in Fig. 1,

Fig. 4 illustrates still another modification of the arrangement in Fig. 1.

' Referring now to the accompanying drawings, wherein like reference characters in the different figures correspond to similar circuit elements, there is shown in Fig. 1 an automatic volume control system for a superheterodyne receiver-particularly It is to be clearly understood that the nature of the receiver with which the volume control network is associated is of comparatively little importance as far as the:

present application is concerned. The superheterodyne type of receiver has been employedas an illustration, because it is at the present time substantially universally employed in broadcast reception in the United States and in many foreign countries. For this reason the various networks of the receiver are conventionally shown in Fig.1, andfit will be understood that the first detectorhas impressed on it energy of local oscillation frequency from a local oscillator. The intermediate frequency output of the first detector network is impressed upon an intermediate frequency amplifier.

The signal input e nergynto the first detector may be connected to asignalcollector, or to one or more stages of tuned radio frequency amplification. The amplified output of the I. F. network is then impressed upon a second detector, or audio demodulator. The demodulated energy is utilizedin one or more stages of audio amplification, and then finally reproduced. Thereceiving system could be employed as a portable receiver in an automobile, or in a cabinet in the home of the user. The operating range of the receiver can be in the broadcast range, or the receiver may be of the multi-range variety, and in that case it could also be operated in the various short wave bands outside the broadcast band.

, The automatic volume control system is utilized in the receiver, as is well known at the present time, in order to maintain a substantially constant energy level at the input of the audio demodulator. Because of fading phenomena, or other causes which result in effects analogous 35 to fading, :the signal energy level at the audio demodulator tends to fluctuate. The automatic volume control system overcomes this tendency, and for this reason it is utilized in modern radio receivers. 40

In Fig. 1, there is shown a form of the invention wherein a fiat AVC characteristic is obtained without the use of high rectified power voltages, background noise suppression also being accomplished. In Fig. 1, the numeral 40 designates a tube of the type, otherwise known as a duplex diode triode tube, the signal input circuit ll of the tube being connected between one of the diode anodes 42 and the cathode of the tube. A load resistor 43 is connected in series between the grounded lead of the cathode of tube 40 and the low alternating voltage side of input circuit 4|. The circuit M is inductively coupled to the circuit 44, which may be connected to an intermediate frequency amplifier, andit is to be understood that the receiving system may be of the type shown in connection with Fig. 1 of my aforementioned parent application. The control grid of tube 40 is connected to a point of negative direct current potential on resistor 43, and the plate of the tube is connected to the positive terminal of a source of voltage B (not shown) through the primary winding of transformer 45. The secondary winding 45" of transformer 45 forms one arm ofa bridge network. 7

The remaining arms of the bridge are the resistor 46, the resistor 46 and the resistor 41. A source of alternating current, which source is not shown to preserve simplicity of disclosure, is connected between two conjugate points. of the bridge, one of these points being at the junction of winding 45" and resistor 46, and the other point being at the junction of resistors 46 and 41. The winding 48 of transformer 48' is connected between the two remaining conjugate points of the bridge. The diode anode 42' of tube 40 is connected to ground through a path which includes lead 49, the winding 48" and the bias resistor 50. A by-pass condenser 5| is connected in shunt with resistor 50, and the AVG lead, which includes the isolation, or filter, resistor 52, is connected from the negative side of resistor 50 to the grid circuits of the controlled tubes. The numeral 53 is to be understood as designating the path through which the audio component of rectified signal energy is impressed upon the audio frequency utilization network.

The usual AVC system requires considerable carrier voltage at the control tube, and the output is not as flat as it should be. When the system shown in Fig. 1 is employed, the usual AVC operation can be improved, and no extra rectified and filtered voltages are required. The signal energy which is impressed on circuit 4| is rectified in the diode circuit, and the latter includes diode anode 42, the resistor 43 and the cathode of tube 48. As the signal voltage increases, the control grid of tube 40 becomes more negative, and this varies the plate impedance of tube 40. bridge circuit, and the unbalance current is stepped up by transformer 48 to give a high voltage. The latter is rectified by the second diode circuit which includes diode anode 42, the resistor 55 and the cathode of tube 40. The rectified output is used to bias the radio frequency and'intermediate frequency amplifier tubes.

The transformer 45 serves to introduce the plate impedance of the tube as one arm of the bridge. This not only permits the bridge to be kept independent of the B voltageybut also permits the resistors in the bridge to be of higher or lower value than the tube impedance, by making the ratio of the transformer right to match the tube into the bridge. The resistor 4'! is shown variable in order to take care of variations with different tubes.

Fig. 2 shows a similar system, with the exception that a magnetic bridge is utilized instead of a resistive bridge. Thus, the numeral denotes the magnetic core of the bridge network, and it will be observed that the winding 45" of transformer 45 is connected to one terminal of bridge winding 6|, and to one terminal of bridge winding 62. The remaining terminal of Winding 6| is connected to the winding 63 of transformer 63; a variable resistor 64 being connected across the winding 63 of transformer 63. The remaining terminal of bridge winding 62 is This plate impedance is one arm of the connected to the remaining terminal of winding 63' of transformer 63. The alternating current source is connected to a point at the junction of windings 45 and 62, and to another point at the junction of windings SI and 63. The diode anode 42 of tube 40 is connected to ground through the remaining bridge winding 65 and the bias resistor 50. The variable resistor 64 functions to adjust the balance of the bridge, and it is to be noted that the windings of the bridge are opposed to each other.

Considering the operation of the systems shown in Figs. 1 and 2, if the bridge network in either arrangement is balanced with no signal applied to the set, the negative bias for the AVG action will go up rapidly as the signal increases in strength. If the tube plate resistance is made slightly lower in value, with no signal, than the opposite arm of the bridge, the bridge will be slightly unbalanced, and there will always be some extra bias applied to the grids of the amplifiers causing the gain to be somewhat reduced. If the signal is strong enough to get through the set at its reduced sensitivity, it will first cause an increase in the plate resistance e of the control tube. This will cause the bridge to become balanced and the bias on the amplifier tubes reduced, and the sensitivity of the set will increase. As a result of this increased gain, the signal will be amplified more, and the bridge will be unbalanced in the opposite direction thus resulting in the initiation of the normal AVC action. The set is thus made less sensitive to very weak signals, and this means that in the absence of a carrier the usual background noise will not be heard. The source of alternating current for the bridge can be any available in the receiver, and may include the high voltage winding, or one of the filament windings, or it can be the line voltage itself.

In Fig. 3, there is shown a modification of the bridge control arrangement, the AVG action being secured by having the signal unbalance an alternating current bridge. This is accomplished by increasing the impedance of a tube, which is one arm of the bridge. The resulting voltage due to the unbalance is rectified, and the direct current voltage component is applied to the control grids of the radio frequency and intermediate frequency amplifiers through suitable filters. Amplified AVC voltage is thus secured, and a delay efiect can be secured by inserting a resistor in the cathode circuit of the bridge tube.

Considering Fig. 3, it will be observed that in this case the bridge comprises resistors 80, 8i, and 82, the impedance of the duplex diode triode tube 83 providing the fourth arm of the bridge. The reactances of condensers 85 and 98 are small compared with the resistance of the bridge arms. Plate voltage is supplied to the plate of tube 83 from a source B, and the three resistors 88, 8|, and 82 are arranged in series between the plate of the tube and the positive terminal of source B. The cathode of tube 83 is grounded through a self-bias resistor 84, the latter being shunted by the by-pass condenser 85, and the magnitude of resistor 84 is chosen to furnish a desired delay action. One of the diode anodes 86 of tube 83 is connected to the high alternating voltage side of the signal input circuit 81 of the system; the latter being coupled to an intermediate frequency amplifier as shown before, if desired, and the low alternating voltage side of circuit 81 is connected to the cathode side of resistor 84 through a path which includes resistor 88.

The control grid of tube 83 is connected to the negative side of diode load resistor 88 through a path which includes the filter elements 89, the latter functioning to remove audio components from the voltage applied to the grid of tube 83. The diode anode 86' is connected to the grounded side of the cathode lead of tube 83 through a grounded resistor 90, and is also connected to the junction of resistors 8| and 8|]. through a condenser 9|. The AVC lead, which is adapted to be connected to the gridcircuits of the radio frequency and intermediate frequency amplifiers, is connected to the anode side of resistor 90 through the filter elements 92, which function to remove hum components from the bias voltage for the controlled amplifier tubes. The alternating current which may be derived from the usual 60 cycle power transformer, or any other point in the receiver, is applied to the bridge network through a transformer 93, the secondary winding 93' of which is connected between the plate of tube 83 and the junction of resistors and 82. The numeral 94 designates the path through which the audio component of the rectified signal voltage developed across resistor 88 is applied to the audio utilization network.

In considering the operation of the system shown in Fig. 3, the intermediate frequency signal voltage is applied to the diode rectifier circuit which includes the diode anode 86 and the load resistor 88. The rectified voltageis used to bias off the triode section of the multi-function tube 83, and this increases the plate impedance of the tube thus upsetting the balance of the bridge. As soon as the bridge is unbalanced, a voltage is caused tobe applied to the second diode circuit which includes anode 86 and load resistor 90. The unbalance current is rectified, and is used as the control voltage for the radio frequency and intermediate frequency amplifiers.

The resistor 84' should be chosen so that when no signal is present, the cathode of the tube 83 will be positive with respect to ground. As the applied signal is increased in amplitude, the plate current of this tube decreases causing the cathode of the tube to become more negative. Until the peak voltage from the unbalance of the bridge exceeds the voltage from cathode to ground no rectification can occur with diode 86. For small unbalances, as when weak signals are being received, no AVC voltage is developed. When large signals are applied, the cathode of the tube may become negative to ground. When this happens, this voltage from cathode to ground will be added to the rectified unbalance voltage to give a large AVC voltage. The regulation of the signal amplitude by this type of AVG will be very effective, because weak signals will cause no AVC voltage, while strong signals will produce a very large AVC voltage.

In Fig. 4, is shown a modification of the bridge arrangement in Fig. 3, and it is to be understood that the system is substantially the same, with the exception of the method of applying the alternating current to the bridge network without the use of a separate alternating current winding. In this, case the transformer 93 is replaced by a resistor 95, and an intermediate point on the resistor is connected to the anodes of the full wave power. rectifier 96 through a condenser 91. Since the rectifier 96 and its associated filter network is of purely conventional design, it is sufficient to point this out and designate the same by the reference character F. The +B side of the filter F is connected to the plate of tube 83 through resistors 82, 8|], and 81, while the negative side thereof is connected to the grounded side of bias resistor 84. In both Figs. 3 and 4, a blocking condenser 98 is disposed between resistor 82 and the grounded side of bias resistor 84.

It is to be understood that instead of employing a duo-diode triode for tube 83, there can be utilized in place thereof a duo-diode pentode. Again, the unbalance voltage may be transmitted through a transformer to step it up before applying it to the diode circuit producing the AVG voltage, and it will be observed that this is suggested in Fig. l, for example.

While I have indicated and described several systems for carrying my invention into effect, it will be apparent to one skilled in the art that my invention is by no means limited to the particular organizations shown and described, but that many modifications may be made without departing from the scope of my invention as set forth in the appended claims.

What I claim is:

1. In a radio receiver of the type including a signal transmission network feeding a signal detector, an automatic volume control arrangement for the receiver comprising a bridge network, a source of alternating current coupled between a pair of conjugate points of said bridge, and alternating current rectifier circuit coupled to the two remaining conjugate points of the bridge, said rectifier circuit including means for developing a direct current voltage from alternating current appearing between said remain: ing two points when the bridge is unbalanced, means for impressing said direct current voltage upon said transmission network as a gain control bias, an electron discharge tube having its input electrodes connected to said signal detector in such a manner that the plate-impedance of the tube varies in response to variations in magnitude of the received signals, and said plateimpedance being included as one arm of said bridge.

2. In a receiver as defined in claim 1, the remaining arms of said bridge being resistors, and said bridge being normally balanced in the absence of received signals.

3. In a receiver as defined in claim 1, said alternating current source being the sixty-cycle power supply source of the receiver, and said signal detector, rectifier and electron-discharge tube all having their electrodes disposed in a common tube envelope.

WINFIELD R. KOCH. 

